The document EP 2602123 A1, for example, describes a hub bearing unit, in this case asymmetrical, for the wheel of a motor vehicle, which includes a flanged hub rotatable around a rotation axis, a flange integral with the hub flanged and transverse to the axis of rotation, a stationary ring disposed radially outside of the flanged hub and provided with rolling tracks axially spaced from one another, and two rolling bodies crowns (for example, balls) arranged between the stationary ring and the flanged hub. The flanged hub integrally form a radially inner raceway for the ball bearing axially outer, while the radially inner raceway for the balls axially inner crown is formed on an inner ring of the bearing, radially outer planted on flanged hub.
Such a realization, especially in the case of heavy duty applications in terms of load transmitted, entails considerable local loads between the bearing rings and rolling bodies; also does not permit to obtain large values of resistance of the bearing and its greater duration in the time. Finally, usually it presents important axial dimensions, due to the presence of a flange integral to the flange portion and the hub transverse to the axis of rotation.
To increase the performance and especially the stiffness of the bearing is required to increase the distance of the pressure centers. This can be achieved by increasing the diameter of the circumference of the centers of the rolling bodies (the so-called “pitch diameter” or more simply “pitch”) of the bearing. Such solutions are already known and are developed in order to significantly improve the performance. The disadvantage connected to the increase of the “pitch” is that consequently also the volume and therefore weight dramatically increase with the “pitch-squared value”. This increase in weight can usually not be accepted by car manufacturers.
A further improvement is to increase even more the diameter of the circumference of the centers of the rolling bodies so as to be able to enter inside the bearing constant velocity joint and integrate in a single piece the so-called bell of the joint with the hub, or with the inner ring of the bearing. Evidently, the integration of both components allows the reduction of weight and cost of the entire unit. And possible to further reduce weight and costs by integrating also the small inner ring of the bearing, the axially internal, with the bell of the joint. In other words, the hub also assumes the function of single inner ring of the bearing and the bell of the joint at a constant speed.
The concept of single inner ring is already known in so-called third-generation bearings. The development of this feature foresaw the use of a single cage for both rolling bodies of the two tracks to be mounted at the axially inner side. This feature, the single cage, however, is not acceptable in current projects of bearing units that require more and more high performance. In fact, the single cage can create several problems. During the operational conditions of the bearing often it happens that the contact angles of the two tracks with the corresponding rolling bodies (for example, spheres) may slightly differ relative to one another. Consequently, they also change the points of contact between tracks and balls and therefore also the diameters of the circumference of the same sphere centers (“pitch”). On an equal angular speed of the wheel, the linear velocity varies proportionally to the “pitch change”. In other words, the two rows of balls have different speed. The cage, which performs the function of keeping the distance between the rolling bodies, however, cannot be subjected to significant forces and as typical consequence, if it is exposed overloads, disintegrates or melts.
It is up to now analyse the case of the so-called symmetrical bearing, that is with the same “pitch” for both crowns of rolling bodies. The drawbacks outlined above, is obviously also present in the case of asymmetric bearings.
Therefore, the existing high-performance solutions, which adopt a single inner ring, require a new mounting procedure, compared to the up to now used for standard applications.